Sinusoidal electrical stimulation at 4Hz enables preferential activation of C-nociceptors in pig and human skin that accommodates during ongoing (1-minute) stimulation. Absence of such accommodation in neuropathic pain patients suggest axonal hyperexcitability that could be predictive of alterations in peripheral nociceptor encoding and offer a potential therapeutic entry point for topical analgesic treatment. Ann Neurol 2018;83:945-957.

BackgroundIntradermal injection of 1 µg nerve growth factor (NGF) causes sustained nociceptor sensitization. Slowly depolarizing electrical current preferentially activates C‐nociceptors.MethodsWe explored the differential contribution of A‐delta and C‐nociceptors in NGF‐sensitized skin using slowly depolarizing transcutaneous electrical current stimuli, CO2 laser heat, mechanical impact, and A‐fibre compression block. In 14 healthy volunteers, pain rating was recorded on a numeric scale at days 1–14 after NGF treatment. Ratings during A‐fibre conduction block were investigated at days 3 and 7 post‐NGF.ResultsPain ratings to electrical, CO2 heat and mechanical impact stimuli were enhanced (>30%, p < .0005, ANOVA) at NGF‐injection sites. Axon reflex erythema evoked by electrical stimulation was also larger at NGF‐injection sites (p < .02, ANOVA). Diminution of pain during continuous (1 min) sinusoidal current stimulation at 4 Hz was less pronounced after NGF (p < .05, ANOVA). Pain ratings to electrical sinusoidal and mechanical impact stimuli during A‐fibre conduction block were significantly elevated at the NGF sites compared to NaCl‐treated skin (p < .05, ANOVA).ConclusionsNGF‐induced sensitization of human skin to electrical and mechanical stimuli is primarily driven by C‐nociceptors with little contribution from A‐delta fibres. Less‐pronounced accommodation during ongoing sinusoidal stimulation suggests that NGF could facilitate axonal spike generation and conduction in primary afferent nociceptors in humans. Further studies using this sinusoidal electrical stimulation profile to investigate patients with chronic inflammatory pain may allow localized assessment of skin C‐nociceptors and their putative excitability changes under pathologic conditions.SignificanceThe application of novel slowly depolarizing electrical stimuli demonstrated a predominant C‐nociceptor sensitization in NGF‐treated skin. Increased pain ratings, larger axon reflex erythema and less accommodation of C‐fibres to ongoing sinusoidal stimulation all indicated an enhanced nociceptor discharge after NGF. A‐fibre conduction block had little effect on electrical and mechanical hyperalgesia skin in NGF‐treated compared to NaCl‐treated skin. This electrical stimulus profile may be applicable for patients with chronic inflammatory pain, allowing localized assessment of skin C‐nociceptors and their putative excitability changes under pathologic conditions.

Slowly depolarizing currents applied for one minute have been shown to activate C-nociceptors and provoke increasing pain in patients with neuropathy. This study examined the effect of transcutaneous slowly depolarizing currents on pruritus in patients with atopic dermatitis. C-nociceptor-specific electrical stimulation was applied to areas of eczema-affected and non-affected skin in 26 patients with atopic dermatitis. Single half-sine wave pulses (500 ms, 0.2–1 mA) induced itch in 9 patients in eczema-affected areas of the skin (numerical rating scale 5 ± 1), but pain in control skin (numerical rating scale 6±1).Sinusoidal stimuli (4 Hz, 10 pulses, 0.025–0.4 mA) evoked itch in only 3 patients in eczema-affected areas of the skin but on delivering pulses for one minute (0.05–0.2 mA) 48% of the patients ( n = 12) reported itch with numerical rating scale 4 ± 1 in areas of eczema-affected skin. The number of patients reporting itch in eczema-affected areas of the skin increased with longer stimulation ( p < 0.005). These results demonstrate a reduced adaptation of peripheral C-fibres conveying itch in patients with atopic dermatitis. Sensitized spinal itch processing had been suggested before in atopic dermatitis patients, and this could be present also in our patients who therefore might benefit from centrally acting antipruritic therapy.